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Host seeking interrupted: behavioural analysis of mosquito responses to bed
nets
J.F. SutcliffeDept. Biology
Blood feeding and disease transmission
-
over 20 arthropod groups have evolved blood feeding behaviour- these include several “true fly”
(Diptera) families, for example:Simuliidae – black flies
Bioimages - UKCulicidae - mosquitoes
USDA
Ceratopogonidae – noseeums
Inst. Animal Health - UK
Psychodidae – sand flies
Univ. Keele - UK
Haematophagous
micropredators–
stalk and locate blood hosts using breath and body odours, heat, moisture and vision- feed on small amount of host tissue (blood)and escape- feed again within a few days on a new host- insect “vectors”
provide an excellent means of transmission for many blood-borne pathogens, e.g., yellow feverdengue, bubonic plague, typhus, malaria…
Mosquitoes –
the world’s most dangerous animal?
http://www.gatesnotes.com/Health/Most-Lethal-Animal-Mosquito-Week
Almost all of thesedue to malaria
Mother and child sleeping in an LLIN
WHO/TDR/Crump
Damaged bed net
Courtesy S. Smith, CDC Entomology Branch
Bed netsLong lasting insecticide treated bed nets (LLINs) are a major strategy for combatting
malaria
-
hundreds of millions of LLINs
have been distributed as part of malaria reduction programs such as the CDC-managed President’s Malaria Initiative (PMI) - over time LLINs
deteriorate physically and lose insecticidal effect-
cost-effective decisions that protect public health must be made about when to replace damaged nets but there are currently no evidence-based methods determine how much damage is too much (= net failure)
Net
DawaPlus
DuranetInterceptor
NetProtect
OlysetPermaNet 2.0
PermaNet 3.0
0
20
40
60
80
100
Mean number of holes per net after 36 monthsKenya
Bed nets work because
Anopheles mosquitoes bite and transmit malaria mainly at night
CDC Public Health Image Bank
#*&%#$^
Host seeking mosquitoes respond to host stimuli and their environment to locate their hosts
Host seekers face many obstacles and vagaries on way to host and
often fail to find host and get blood meal
Bed nets are just one more obstacle faced by the host seeking mosquito. How do they respond to this obstacle?
Extensive insectary
facilities includeinclude climate controlled rooms forlarge scale experiments and manymosquito species in colony
Three questions addressed:
Question 1) How likely are mosquitoes to go through holes of different sizes and shapes in the net?
Question 2) What parts of the occupied bed net are under the greatest ‘mosquitopressure’
-
where are the mosquitoes most likely to try to get in?
Question 3) How can we combined these findings into a model predicting vulner-ability of damaged bed nets?
-
Mosquitoes fly/skim close to the net surface when searching for holes
search occurs largely in two-dimensions
-
Searching mosquitoes may come across holes in the net but not pass through
hole passage
must be preceded by hole encounter
but doesn’tnecessarily follow it
Therefore, the probability (P) of a mosquito entering net through a hole = P(encounter) X P(hole
passage)
Question 1) How likely are mosquitoes to pass through holes of different sizes and shapes in the net?
Lessons learned from previous observations
Experimental systemFibreglass
screen “bed net”
system in3mX3m containment tent:- smooth flat surfaces for video-
interchangeable system for switching different holes in and out-
fibreglass
screen accommodates various sizes and shapes of holes in material that does not ravel- experiments done in darkened room
3) 30cm X 30cmsampled area
1) position line laser to project along outside about 1 cm away from net to capture mosquitoes when against the net2) position another line laser to project close to the inside of
net to capture mosquitoes as they pass through the hole 3) define a 30cm X 30cm sampling area around pre-cut net hole (corresponds to 30cm X 30cm sampling area of sticky squares) 4) place camera(s) to record activity on net near hole(s)
Configuring the fibreglass
net for video observations
1) green laser source outside
net2) red laser source inside
net
fibreglass
screeningin aluminum framesforms walls and roof
4
1cm 1cm approx. 1cm
inside the net
outside the net
red laserfield
green laserfield
bed net
edges of net hole
Net seen end-on
line laserprojects along
net front
line laserprojects across
net roof
Laser fields withnet between.
Mosquito inside net
Example of video observationsRight -
12mm X 200mm hole in side of net (taped over before experiment to prevent premature mosquito entry)
Below, left and right -
Screen grabs of experimental video illustrating hole (outline) and mosquito: 1) “appearance”(two mosquitoes), 2a) “encountering”
hole and 2b) “entering”
hole
1
2a
2b
What hole feature best predicts encounter probability?
- various candidates-
hole area-
hole width-
hole circumference
- hole circumference is the bestpredictor of hole encounteraccounting for almost 75% ofvariation in dependent variable
What hole feature best predicts hole entry
probability?
hole width, irrespective of other hole measures (e.g. length, area, circumference, etc.), predicts almost 83% of the variability in per encounter entry probability
0
90
180
270
360
450
0
2
4
6
8
10
12
14
16
18
20
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96
18‐20
16‐18
14‐16
12‐14
10‐12
8‐10
6‐8
4‐6
2‐4
0‐2
Hole width (mm)
Per
mos
quit
o P
of g
etti
ng in
to n
et
Hole cir
cumfer
ence (
mm)
0.18-0.200.16-0.180.14-0.160.12-0.140.10-0.120.08-0.100.06-0.080.04-0.060.02-0.040.00-0.02
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Estimating P(of
getting into the net) through holesof different sizes (estimating hole “passability”)e.g. 1) for 12mm wide X 150mm circum. hole, P = 2-4%
2) for 30mm wide X 220mm circum. hole, P = 4-6% 3) for 90mm wide X 360mm circum. hole, P = 10-12% 4), 5), 6)…
How many mosquitoes actually enter the net also depends on whether mosquitoes are found on the parts of the net where the holes are
Question 2
12
6
4
5
3
Question 2) What parts of the occupied bed net are under the greatest ‘mosquito pressure’
-
where are the mosquitoes
most likely to try to get in?
All areas more or less the same?
Question 2) What parts of the occupied bed net are under the greatest ‘mosquito pressure’
-
where are the mosquitoes
most likely to try to get in?
To the feet (smelly socks effect) or to the head (exhaled breath
effect)?
Question 2) What parts of the occupied bed net are under the greatest ‘mosquito pressure’
-
where are the mosquitoes
most likely to try to get in?
Or do they follow a heavier-than-air-CO2
plume?
10’
by 10’
REI containment tent and bed net
Bed net with sticky screen squares
Bed net divided into 18 sampling areas
Experimental set up
Containment tent(3mX3mX2.5m)
Bed net (1.8mX1.3mX1.5m) on
frame hung with sticky squares (not shown)
Subject in bed net
Experimental set up (cont’d)
Up to 200 hungry female An. gambiae mosquitoes from CDC colony released into tent
Experimental set up (cont’d)
J
Data presentation –
bed net folded out and flattened pictorially to depict results as a bubble chart
0.01
0.01 0.01 0.00
0.000.01
0.02 0.01 0.01
0.010.08
0.09 0.08 0.08
0.110.04 0.42 0.01A
NML
IHG
DCB
OJERQFK P
Generic layout of whole net capture dataEach lettered square corresponds to asector on net
end (head) top
side
end (feet)
Hypothesized mosquito movement pattern around an occupied bed net (human subject not shown).
Sutcliffe and Yin Malaria Journal 2014 13:294
‘X’
–
mosquitoes re-leased‘A’
–
fly along floor, hit net, fly up‘B’
–
encounter hostodour plume, dropdown to roof, andto floor‘C’
–
fly along floor,hit net or tentsides, fly up again‘D’
–
encounterHost plume, etc.
Question 3) How can we combine these findings into a model predicting vulnerability of damaged bed nets?
Use:1) passabilities
based on hole width and circumference:Hole 1) 12mm wide X 150mm circum. hole, P = 2-4% Hole 2) 30mm wide X 220mm circum. hole, P = 4-6%Hole 3) 90mm wide X 360mm circum. hole, P = 10-12%
and2) mosquito densities
mosquitoes measured with sticky squares on the net:
to run some simple hypothetical scenarios
Number of mosquitoes entering the net per unit time = Hole passability
X Density of mosquitoes near the hole
(totalled for all holes in the net)
1
10
100
1.0 –
1.2
2.0 –
2.4Total =
Relative numbers of mosquitoes entering net/hour (assuming 200 present)
Net scenario A –
two hole 3s (90mm wide X 360mm circum.)in upper two thirds of side
-
surprisingly few mosquitoes enter despite two large holes in net
1.0 –
1.2
Mosquitoes entering
20.0 –
40.0
1.0 –
1.2
4.0 –
6.0
25.0 –
47.2Total =
Relative numbers of mosquitoes entering net/hour (assuming 200 present)
Net scenario B –
size 1 hole in roof, size 2 hole in bottom third of side,size 3 hole upper third of side
-
smallest hole contributes over 80% of mosquitoes because of its
locationon the roof- largest hole contributes less than 5% of mosquitoes because its
in a location where mosquitoes are sparsest
Mosquitoes entering
Conclusions/recommendations
-
even one small hole on the net roof may render a bed net non-protective
-
nets with large holes may still offer protection depending on where the holesare located and the overall density of mosquitoes
- net vulnerability can be relatively easily estimated by collecting informationabout:
-
number of holes-
locations of holes - widths of holes-
circumferences of holes
Part of current protocols?YesNoNoNo